1. Field of the Invention
The present invention relates to a method for producing an optical fiber preform.
2. Description of the Related Art
Optical fibers, largely used in optical telecommunications, are typically made by fusing and drawing an optical fiber glass preform. There are known different techniques to produce an optical glass preform, the main being outside vapor deposition (OVD), modified chemical vapor deposition (MCVD), vapor axial deposition (VAD), and plasma-enhanced chemical vapor deposition (PECVD).
Each of the above techniques conventionally involves: i) delivery of a vapor flow containing glass forming precursors to an oxidation site such as, for example, the flame of a gas/oxygen burner or hot plasma zone adjacent to a deposition substrate or inside a deposition tube; ii) oxidation of the vapor flow to form a particulate or soot oxidation product; and, iii) collection of the particulate or soot oxidation product on the substrate or tube to form a preform (in the PECVD process, the glass is deposited directly from the vapor phase onto the tube without the intermediate soot formation step). While in the MCVD and PECVD processes the resulting soot preform is generally clear after the deposition stage and can be drawn into fiber without a sintering step, in the OVD and VAD methods the resulting soot preform is further processed, by sintering, to form clear glass from which an optical waveguide fiber is drawn. Dopants may be included in the vapor flow to modify various characteristics of the resulting glass such as refractive index or coefficient of thennal expansion.
Silica (SiO2)-based optical fibers have long been commercially preferred for optical telecommunications. By providing a preform with a radially varying refractive index profile, an optical fiber with the requisite waveguiding characteristics can be drawn. In order to provide the appropriate waveguiding characteristics, SiO2 has been doped with various compounds to alter Its refractive Index. These compounds include, for example. GeO2, TiO2, Al2O3 and P2O5. Vapors containing these compounds are conventionally provided using metal halides such as GeCl4, TiCl4, and POCl3 . It Is also known that certain compounds, such as rare earth elements, can be incorporated into the glass structure to provide other optical-functions including lasing and signal amplification.
In a conventional OVD process, silica soot is deposited on successive layers onto a central bar, usually named “core”or “mandrel”. For purposes hereof, a layer is defined as that portion of glass soot that is deposited by one pass of the burner along the mandrel. Silica is generated in the above-mentioned oxidation of the vapor flow. Deposition is typically performed by reciprocating the burner in parallel to the mandrel, while the mandrel rotates about its axis. This deposition process is typically performed twice: the first time to produce an intermediate preform (whose material will give rise to the core and to an internal part of the cladding of the fiber) on a removable mandrel, and the second time (in the so called “overcladding step”) for depositing a cladding soot (which will give rise to an external part of the cladding of the fiber) on a mandrel obtained by stretching and cutting the intermediate preform. The overcladding step provides the majority of the material that will constitute the optical fiber and then it requires much more time than the first deposition step. At the end of the two deposition steps, a final preform is obtained, which is successively consolidated and drawn into an optical fiber.
Soot density during the deposition process is an important parameter to achieve a final glass preform of good quality. The deposition density is related to the temperature of the substrate on which the soot is deposited, the higher being the temperature the higher being the density.
Several documents deal with density of a soot preform in a chemical deposition process.
U.S. Pat. No. 6,050,108, in the name of Sumitomo Electric Industries, although dealing with a VAD process and, in particular, to the stages of degassing and consolidating the preform, states that the porous glass preform should preferably have a bulk density not less than 0.6 g/cm3, more preferably from about 0.6 g/cm3 to about 0.8 g/cm3. If the bulk density falls below this range, the porous glass preform tends to be broken because it is too soft. On the other hand, if the bulk density exceeds the above range, the glass preform has too much high hardness and therefore, air bubbles already Incorporated therein are hardly removed and tend to remain.
U.S. Pat. No. 4,810,276 in the name of Coming Glass Works describes an OVD process for making an optical fiber preform, wherein the first layer of cladding soot is deposited on a core glass bait rod at a density of at least 0.5 g/cm3 {preferably in the range of about 0.6 g/cm3 to 0.7 g/cm3}, while the remainders of the layers are deposited at the same density or at a density gradually decreasing from the density of the first pass as radius increases. According to U.S. Pat. No. 4,810,276, depositing the soot on the bait rod at a density less than about 0.5 g/cm3 determines a bond of the soot particles to the core surface that is not strong enough to force conformity of the core to the soot coating as the coating shrinks longitudinally during consolidation. To obtain the said soot density in the first layer, the rod is preheated by a burner and an auxiliary burner immediately prior to soot deposition.
The Applicant observes that the solution of pre-heating the bait rod proposed in U.S. Pat. No. 4,810,276 allows only controlling the density in the first layer, while there is no suggestion on how to control the soot density in the following layers. Moreover, an auxiliary burner is required for pre-heating the bait rod.
U.S. Pat. No. 4,627,867, in the name of Sumitomo Electric Industries, faces the problems of soot rod cracking or lowering of deposition yield when the temperature of the soot rod during deposition is respectively too low or too high, and proposes to deposit fine glass particles by means of a first burner at such a temperature that a soot rod at a low bulk density is formed, and subsequently heat the soot rod by a second burner so as to increase the bulk density of the soot. The soot rod is then sintered to obtain a transparent glass preform for an optical fiber. In a preferred embodiment, firstly the soot rod having the low specific bulk density of from 0.02 to 0.1 g/cm3is formed and then the bulk density is increased to from 0.15 to 0.5 g/cm3.
As previously, a second burner is required, which adds complexity and cost to the system. Moreover, this technique leads to an increase in the gas consumption.
U.S. Pat. No. 4,731,103, in the name of Sumitomo Electric Industries, describes a method for producing a glass preform for an optical fiber by the OVD method, by which a glass preform having desired distribution of bulk density and not suffering from cracking or the fluctuation of the additive concentration In its radial direction is produced. This method comprises measuring the temperature of a part of the soot rod on which the glass particles are deposited and controlling said temperature by adjusting the fuel gas-jetting rate. The soot rod is then sintered to obtain a transparent glass preform.
A similar technique is proposed by JP 04367536 A2, in the name of Fujikura. This patent relates to a method for manufacturing a rare earth-added quartz and faces the problem of non-uniform impregnation of a soot body dipped in a rare earth metal alcohol chloride solution, when the soot body has a bulk density in the center section higher than in the surface section. This problem is solved by producing a soot body by an OVD method, in which the deposition of the soot is performed while the surface temperature of the outer circumferential surface of the soot body to be deposited on a rod-shaped starting parent material is kept constant and, more particularly, by providing a soot body with an average bulk density of 0.3 to 0.7 g/cm3 and a bulk density change in the radial direction within ±0.5%. For this purpose, an infrared thermometer is used to measure the surface temperature of the soot body, and a flow controller controls the flow of hydrogen and oxygen supplied to the burner in accordance with a temperature signal from the thermometer. The described process also comprises the withdrawal of the burner in the perpendicular direction with respect to the parent starter material, in accordance with the extent of growth of the body of the soot, in such a way that the surface temperature of the outer circumferential surface of the soot body is maintained at a constant.